This invention relates to the art of displacement sensors and, more particularly, to an interferometric displacement sensor employing fiber optics for detecting displacement changes of a test article.
The invention is particularly applicable for use in conjunction with determining displacement changes of a test article subjected to displacement due to thermal expansion, such as during thermal vacuum testing of space vehicle components where knowledge of thermally induced distortion is required and the like.
The prior art has employed fiber optic technology for use in strain gauges. One example of such prior art is found in A. Harmer U.S. Pat. No. 4,163,397. This patent discribes a structure for measuring strain in a solid object employing an elongated optical fiber having a curved form which is attached to the solid object in such a way that the stresses are translated into changes of the curve form. This, in turn, creates changes in the propagation of light passing through the optical fiber. Harmer's apparatus, then, depends on changing the bend in the optical fiber to thereby create changes in the modal propagation of light therethrough. Contrary to the invention described therein, Harmer does not propose that the fiber be stretched as the object under test is lengthened, or that the dimensional change be measured, as with an interferometric device. Moreover, Harmer's apparatus serves as a strain guage, as opposed to the displacement sensor of the present invention.
Another teaching in the prior art relative to optical strain gauges takes the form of C. Butter U.S. Pat. No. 4,191,470. Butter discloses a laser-fiber optic interferometric strain gauge. The apparatus directs laser light into two single mode fibers, which are, in turn, each attached to a supporting member, the strain of which is to be measured. The far ends of the fibers are brought into close proximity and the light exiting from the two fibers interferes. As the supporting member undergoes a bending movement, it is strained and the interference fringes move providing a measure of the strain. It is to be noted, however, that Butter secures both of the fibers to the supporting member and the measure obtained is the difference of the length changes of the two fibers and not that of the supporting member itself.